U.S. Pat. No. 3391326 (Penberthy) describes and claims a connection between and electrical conductor and a refractory ceramic electrode for use in the electrical heating of molten masses, such as glass, wherein contact between the metal conductor and the ceramic electrode is made by means of a metal which has been made molten to wet into good electrical contact with the electrode. The electrode is in practice made of moulded and fired tin oxide. The practical forms of the Penberthy electrode are neither advanceable nor extendible, and since a tin oxide electrode is subject to erosion, it must be replaced a number of times during the service life ("campaign") of the furnace. A structure for a tin oxide electrode that is said to be applicable to both vertical and horizontal electrodes and not to rupture or break during continuous operation is described in U.S. Pat. No. 3826858 (Pieper) and employs a contact cup that is pushed against the end of the electrode by means of a ram. An end bored ceramic electrode structure fitted with a contact plug in the form of a sleeve of silver filled with a plug of the same ceramic material as the electrode is described in U.S. Pat. No. 41105545 (Shaw et al).
Patent Specification No. GB-A-1481406 (Elemelt) describes a rod-type electrode of tin oxide made in parts that interfit together endwise with optionally connection between adjacent sections being established by silver or silver alloy, but the individual sections of electrode are solid and there is no provision for a metallic current path from base to adjacent the working tip of the electrode. The Element electrode is not stated to be advanceable or extendible.
Tin oxide electrodes constructed according to the principles described in Pat. No. 3391237 (Penberthy) have been used by Penelectro/KTG since the late 1960's in electric furnaces for melting lead crystal-type glasses. These furnaces generally make not more than 35 tons of glass per day, and most of them make not more than 15 tons per day. That there is a barrier to the increase in size of an individual furnace will be apparent from the fact that there is in excess of 750 tons of electrically melted lead glass produced per day.